Cross-counter time-delay distributions at the Mawson neutron monitor

Abstract

For decades, neutron monitors have been the primary source of ground-level data for studying cosmic rays with rigidities slightly above the geomagnetic cutoff. The NM64 instrument design, the standard for neutron monitors, has been optimized to monitor neutrons produced in atmospheric cascades. It includes thick slabs of polyethylene to reflect low-energy neutrons, a large mass of Pb to multiply signals at high energies, and an array of cylindrical, gas-filled proportional neutron counters. Mawson station, located in the Australian Antarctic Territory, has one of the longest continually operated polar monitors. In 2020, the station’s neutron monitor was upgraded to allow the recording of timing information for individual signal pulses. Also at that time, an extra array of 6 bare counters (with no Pb or polyethylene reflector) was installed in addition to the 3 standard sections of 6 counters each. We analyze the cross-counter time-delay distributions to study timing correlations between pulses on different counters. The results show the expected dependence with tube separation within each NM64 section and clear correlations between different sections and between NM64 and bare counters, which can be at relative distances of up to 10 meters. The cross-counter time-delay distributions may thus be used to study the structure and size of atmospheric cascades. The timing information recorded by the upgraded monitor provides a new dimension of data for researchers to analyze and gain insight into the complex processes of cosmic ray propagation and interaction with Earth’s atmosphere.